1. Field of the Invention
This invention relates to a tip and substrate preparation system for use with scanning probe microscopes (SPMs) comprising a tip maker, methods for coating scanning tunneling microscope (STM) tips for electrochemical use, a substrate treatment method for producing clean, flat gold substrates for STM use and methods for preparing chemically activated substrates for use with an atomic force microscope (AFM). The tip maker includes a coater and an etcher which are preferably controlled by electronic controllers. The etcher provides fully automatic tip etching in a two-stage process in sodium hydroxide (NaOH) solution, permitting platinum alloys to be etched without the use of cyanide-containing chemicals. The coater is used to insulate the tips with soft polymer coatings so as to ensure very low tip leakage current (on the order of about 1 pA typical). The substrate treatment device comprises a quartz plate and a quartz torch for annealing substrates in a hydrogen flame. The chemically activated substrates for atomic force microscopy permit the surface of mica to be modified at will so as to be hydrophobic, hydrophilic, positively or negatively charged.
2. The Prior Art
The probe tip is the most critical element in a scanning tunneling microscope. It is usually made from a tungsten (W) or a Platinum-Iridium Alloy (Pt--Ir) wire. The sharpness and stability of the tip affects the quality and resolution of the STM image of the surface directly. In recent prior art (see, e.g., I. H. Musselman, et al., "Platinum/iridium tips with controlled geometry for scanning tunneling microscopy", Journal of Vacuum Science and Technology, Vol. A 8(4), pp. 3558-3562, 1990; J. P. Ibe, et al., "On the electrochemical etching of tips for scanning tunneling microscopy", Journal of Vacuum Science and Technology, Vol. A 8(4), pp. 3570-3575, 1990; and A. Cricenti, et al., "Preparation and characterization of tungsten tips for scanning tunneling microscopy", Review of Scientific Instruments, Vol. 65, No. 5, pp. 1558-1560, May, 1994), tips are prepared by electrochemical etching of W or Pt--Ir wires. For imaging in ambient and fluid environments, a W tip suffers from the formation of an oxide layer on its surface. Pt--Ir alloy is thus preferred because of its relative chemical inertness. However, making Pt--Ir tips formerly involved the use of a solution containing sodium cyanide (NaCN), an extremely toxic and regulated chemical which, aside from being dangerous, presents onerous requirements for use which are difficult to conveniently meet in a typical university or industrial laboratory setting. Other methods for etching Pt--Ir tips include the use of CaCl.sub.2 /H.sub.2 O/concentrated HCl solution and molten salts. Such methods require a relatively long etching time (on the order of about 20 minutes about minutes) and a complex mixture of chemicals. As will be seen shortly, these problems are solved by the present invention.
There are two commonly used methods for preparing Pt--Ir tips: (1) one step drop-off method, and (2) two step bulk etching/micropolishing method. The first method is summarized in the article of L. A. Nagahara et al., "Preparation and characterization of STM tips for electrochemical studies", Review of Scientific Instruments, Vol. 60, No. 10, pp. 3128-3130, October, 1989. The basic setup consists of a beaker containing an electrolyte (typically 3 M NaCN and 1 M NaOH) and a vertical adjustment to control the depth of immersion of a platinum (Pt) alloy wire (usually Pt--Ir). A piece of Pt--Ir wire is dipped into the electrolyte near the center of the beaker using the vertical adjust. The length of the immersed part of Pt--Ir wire is adjusted to give an initial etching current value of 0.5 A. A circular nickel (Ni) foil placed in the beaker is used as counterelectrode. During the etching process, the section at the air-solution interface and the extreme lower end of the wire are etched much faster. Thus, when the neck of the wire near the interface becomes thin enough, it is fractured by the weight of the wire in the electrolyte. The etching is terminated when the lower part of the wire drops off. The most important parameter affecting the final shape of the tip end is the time delay in removing the applied voltage after the lower part drops off. An electronic circuit is often used to sense the abrupt decrease of the etching current which accompanies the drop-off. It then turns off the applied voltage after a preset delay. However, reproducible tip shapes are nearly impossible to achieve because of the noise level in the current due to fluctuations associated with the turbulence in the fluid during etching. A second method consists of two steps: bulk etching followed by micropolishing. The bulk etching is carried out as described previously. The wire is etched in bulk solution to obtain the overall shape required and a precision micropolishing is then done in a thin film of etchant held in a wire loop so as to be positioned over the apex of the tip. The loop has to be raised and lowered with a mechanical micropositioner to achieve best tip shape. The whole process of making a tip is long and complex, requiring much practice and training. The present invention solves these problems by automating the entire etching process through a controller, which uses a phase-locked circuit to detect the etching current very accurately and terminate the etching promptly.
Obtaining a sharp tip is essential for ensuring a high quality and high resolution images of a surface, but operating an STM in electrochemical environments also requires that the STM tip be well insulated, with just a small protrusion of bare metal at the very apex of the tip. Good insulation reduces the Faradaic leakage current and, consequently, noise. An ideal tip for this environment should have a chemically and electrochemically inert insulation except for the very end of the tip, which should be uncoated to allow electron tunneling to occur. In the prior art, Glass-coated and Poly(.alpha.-methylstyrene)-coated tips have been used. However, both of these materials are brittle and non-ductile and therefore crack easily, resulting in a reduction in their insulation capabilities and an increased Faradaic current. In addition, glass-insulated tips cannot be used in concentrated alkali solutions because they will dissolve. SiO.sub.2 evaporated onto glass-coated tips has also been tried with good results, but this process is time consuming and requires an expensive high vacuum coating system. In recent years, Apiezon wax has been used as an alternative insulating material and gives satisfactory results (See, e.g., L. A. Nagahara et al., supra). In recent prior art for using wax as an insulating material, an etched tip is mounted vertically on a manipulator, and brought underneath a wax holder. The tip is poked through the molten wax, brought out of the wax and tested for leakage. The typical leakage of these wax-insulated tips is about 100 pA. This process is very slow and often has the problem of damaging the apex of tips with thermal shock because the tip has a much lower temperature than the wax when it enters the wax. This problem is avoided in the present invention, both by the use of better coating methods and through the use of alternative coating materials. Furthermore, Apiezon wax is soluble in many organic solvents, such as toluene and benzene, and therefore cannot be used in non-aqueous electrochemistry. This problem is solved in the present invention as well.
A flat and clean substrate is very important for imaging biopolymers and other adsorbates under electrochemical potential control in an electrochemical scanning probe microscope (ECSPM). In recent ECSPM development, the most commonly used substrates have been gold (Au) single crystals, or Au films evaporated onto mica and annealed in an ultra high vacuum (UHV) for many hours. Fabricating such substrates requires costly setup and time consuming procedures. Furthermore, these procedures do not yield clean substrates reproducibly due to contamination when the vacuum system is first re-pressurized or opened. A reliable and easy-to-use alternative method for preparing substrates is required. Single crystals of gold or gold films evaporated onto a substrate that is stable at high temperature may be cleaned and flattened with the use of a hydrogen flame, as is well known in prior art for forming solid electrode surfaces for electrochemistry. However, neither of these surfaces produces flatness comparable to gold evaporated onto mica. It has generally been assumed that mica cannot be annealed with a hydrogen flame because it breaks down at temperatures above 500.degree. C.
In atomic force microscopy, the substrate is not required to be conductive and mica is commonly used because it yields large atomically-flat areas easily. However, the surface of mica is rather inert, and not many materials stick to it well. It must be treated in order for it to bind many types of molecule. Lyubchenko et al., "Atomic Force Microscopy Imaging of Double Stranded DNA and RNA", Journal of Biomolecular Structure & Dynamics, Vol. 10, No. 3, pp. 589-606, 1992, have described a mica surface treatment with 3-aminopropyltriethoxy silane. This treatment binds amine groups to the mica surface. They become protonated in water to give the surface a positive charge so that it attracts negatively-charged molecules such as DNA. It would be more useful to be able to place a very reactive group onto the mica surface so that it could be modified at will so as to bind positively charged, hydrophobic, and hydrophilic molecules and molecules with specific reactive sites.
There has been rapid growth of the use of chemically functionalized AFM force-sensing probes [C. D. Frisbie et al., "Functional Group Imaging by Chemical Force Microscopy", Science, Vol. 265, pp. 2071-2074, Sep. 30, 1994]. The chemistry that is used to modify mica can also be used to modify force probes made from silicon compounds.
In short, while the microscopes for ECSPM are being developed rapidly, tip and substrate preparation systems are not generally available as efficient, easy to use laboratory tools.